Support IsolationMode and RunAs

[daaru00]

Add support of IsolationMode and RunAs configuration.

Configuration proposal (defaults):

custom:
  greengrass:
    groupId: 'xxxxxxxxxxxxxxxxxxxxxxx'
    defaults:
      isolationMode: 'GreengrassContainer'
      runAs:
        uid: 1
        gid: 10

Configuration proposal (defaults with override):

custom:
  greengrass:
    groupId: 'xxxxxxxxxxxxxxxxxxxxxxx'
    defaults:
      isolationMode: 'NoContainer'

functions:
  myfunction:
    handler: tasks/dummy/task.handler
    greengrass:
      isolationMode: 'GreengrassContainer'
      runAs:
        uid: 1
        gid: 10

Default behavior can be GreengrassContainer. Default value can be thread at DefaultConfig property instead of assign it to every functions.

[revmischa]

Would be awesome. MemorySize and AccessSysfs need to not be set in FunctionDefinition if mode is NoContainer